Experimental details

1. Subjects Number: ten per experimental run.
Experimental runs: four.
Age: 20 to 31 years.
Sex: male.
Racial origin: Maya Indian and Spanish.

TABLE 1. Ascending Protein Intake Sequence

*Rice and bean (60:40 protein distribution) in study I with 45 kcal/kg/day; in study 2 with 50 kcal/kg/days; in study 3 with 10 per cent milk protein substitution with 45 kcal/kg/day and in study 4 with 10 per cent milk protein substitution with 50 kcal/kg/day.

Physiological status: normal.

Nutritional status: acceptable-weight 49.1 to 65.0 kg; height 157 to 172 cm.

Health status: free of chronic infections and free of intestinal parasites.

2. Study Environment Location:
Metabolic Unit of the Division of Food Science, INCAP. Climatic characteristics: temperature 21 to 25 C (day); relative humidity 77 to 85 per cent; altitude 1,470 m above sea level; months of September to December 1979.

3. Physical Activity Normal
(laboratory technicians and institution maintenance crew).

4. Duration of Study
Total time per Study nine days.

Procedure: short-term, multiple-point nitrogen-balance assay.

Protein intake changes: every two days, as indicated in table 1.

TABLE 2. Nitrogen-Free Diet Composition per Assay

1,3 Prepared from wheat starch (Jolly Joan, Ener-G Foods, Inc., P.O. Box 24723, Seattle, Washington 98124, USA).
2 Prepared from cornstarch and margarine, and seasoned with aromatic herbs. Herbs were not consumed
4 UNIT TMR.

5. Diets Table 2 describes the ingredient composition of the basal diet fed (nitrogen-free diet), and table 3 shows the protein content of the rice and beans used in the four studies. Table 3 also describes the vitamin and mineral composition.

The protein and energy intake from rice and beans was calculated every two days. It was given to the subjects in three equal portions, or during lunch or dinner. Large batches of raw rice and beans were purchased to reduce the variability that could be caused by differences in quality. The beans were cooked in large lots by soaking for 14 to 16 hours followed by cooking at 151bs/inch2 (1.05 kg/cm2) for 30 minutes. The material was then stored frozen. Rice was steamed.

TABLE 3. Protein Content of Protein Sources

*Fresh basis.

6. Measurements and indicators
Composition of diets: AOAC methods of proximate chemical analysis.

Digestibility: apparent and true.

Nitrogen balance: apparent-does not include other losses.

Indicators of protein quality and utilization: (a) linear regression analysis (y = a + bx) of nitrogen intake (Nl) to nitrogen retention (NR) and of nitrogen absorption (NA) to NR; N i for N R = 0; (b) quadratic regression analysis (y = a + bx + cx²) of Nl to NR and of NA to NR, used primarily to estimate recommended protein intake as obtained by calculating the first derivative dy/dx = b + cx, where x (Nl or NA) is equal to-(b/2c).

Energy: digestibility and metabolizable energy at 0.6 9 protein intake and by measuring energy in food, faeces, and urine.

Other determinations and measurements: none.

Summary of Main Results Protein digestibility (apparent) of the rice and bean diet (60:40) was not affected by a difference in energy intake of 45 to 50 kcal (59.1 vs. 59.6 per cent). It was increased by the 10 per cent milk replacement of the protein in the rice and bean mixture without being affected by energy intake (65.3 and 64.6 per cent). The protein digestibility of milk was 75.6 per cent. Energy digestibility of the diets varied from 93.3 to 94.6 per cent, while metabolizable energy varied from 91.9 to 92.9 per cent.

The linear coefficient of regression between Nl and NR was not affected by energy intake without milk (0.75 and 0.79) or with milk (0.95 and 0.86), but milk supplementation improved it significantly, and was no different from milk alone (0.91). The Nl for NR = 0 followed the same trend. For 45 and 50 kcal/kg/day the values without milk were 95.6 and 92.9 mg/kg/day, and with milk, 78.4 and 80.8 mg/kg/day. The milk reference value used was 86.6 mg/kg/day. The relative coefficients of regression to milk equal to 100 per cent were 82.4, 86.8, 104.4, and 94.5 per cent.

The effect of milk was attributed to an improvement in protein digestibility rather than to an improvement in essential amino acid balances, as judged by amino acid content and by the linear regression coefficients of NA to NR, which were statistically alike (0.99, 1.02, 1.10, and 1.04 for diets, and 0.91 for milk). Using the quadratic regression equations, the protein intakes for maximum nitrogen retention were 0.79, 0.79, 0.69, and 0.74, with diets of better quality giving lower values. Correcting for differences in digestibility, ail values were similar, with an average of 74.8 mg N/kg/ day. This value was interpreted to represent the amount needed for all of the population to be in positive and maximal balance.